Controlled growth of fatigue cracks by electromagnetic means



Dec. 9, 1969 c. H. MARTENS 3,482,437

CONTROLLED GROWTH OF FATIGUE CRACKS BY ELECTROMAGNETIC MEANS Filed July 26, 1967 POWER SOURCE Charles H. Martens,

INVENTOR.

BY $7 "f Wz Z! ATTORNEYS /7 agm-A enf I United States Patent 3,482,437 CONTROLLED GROWTH OF FATIGUE CRACKS BY ELECTROMAGNETIC MEANS Charles H. Martens, Huntsville, Ala., assiguor to the United States of America as represented by the Secretary of the Army Filed July 26, 1967, Ser. No. 656,636 Int. Cl. G01n 3/00 US. C]. 73-91 7 Claims ABSTRACT OF THE DISCLOSURE Controlled fatigue cracks are grown in structural materials by electromagnetic means. The apparatus used for inducing a crack in a discrete area of the structural material surrounds and contacts the structural material around the discrete area. An electromaguet intermittently magnetizes the apparatus that surrounds and contacts the structural material and thereby induces a crack in the discrete area of the structural material by exerting forces on the material in the discrete area.

BACKGROUND OF THE INVENTION This invention relates to an apparatus for inducing fatigue cracks in structural materials and particularly to such an apparatus wherein electromagnetic rneans are employed.

Investigators of the mechanical properties of structural materials are increasingly intentionally growing fatigue cracks of predetermined characteristics in structural materials. The controlled growth, to date, of such cracks has been difiicult and dangerous. The difficulties are high cost and slowness of operation. The danger is primarily to the operator in the event of uncontrolled structural failure. However, danger also exists to the surrounding equipment and room.

Accordingly, the principal object of this invention is to provide an improved apparatus for growing controlled fatigue cracks in various structural materials.

A particular object of this invention is to provide a safe apparatus for growing controlled fatigue cracks in various structural materials.

SUMMARY OF THE INVENTION The apparatus of this invention achieves the growth of controlled cracks in structural materials by electromagnetic means. The apparatus of this invention used for inducing a crack in a discrete area of the material surrounds and contacts the structure material around the discrete area by being contoured to the contour of the structural material. An electromagnet is used to intermittently magnetize the apparatus that surrounds and contacts the structural material and thereby induce a crack in the discrete area of the material by causing forces to be exerted on the material in the discrete area.

BRIEF DESCRIPTION OF THE DRAWING These and other objects and advantages of this invention will become rnore readily apparent from the following detailed description of this invention, of which the accompanying drawing forms an integral part. In the drawing, the single figure is a diagrammatic view of the apparatus of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The apparatus of this invention achieves the growth of controlled cracks in structural materials by electromagnetic means. Referring now to the figure, 1 is a structural material to be tested. A crack starter notch 2 has preferably been placed in structural material 1 previous to 3,482,437 Patented Dec. 9, 1969 up strip 3 must be used if structural material 1 is nonmagnetic. The use of a magnetic material backup strip 3 is optional with a magnetic structural material. Of course, the contour of the magnetic material backup strip 3 is compatible with the matching surface of structural material 1. Magnetic material backup strip 3 must have a smaller contact area than the discrete area around crack starter notch 2 on the opposite surface of structural material 1.

An electromagnetic core 6 has a tip 4 that is shaped and contoured to contour of the structural material it is to contact and is illustrated in the drawing as being in contact with the upper surface of structural material 1, thereby defining a discrete area around crack starter notch 2. Tip 4 is preferably .detachable from core 6. However, if desired, tip 4 can be made of integral construction with core 6. An indentation 5 is shown in tip 4. The depth and contour of indentation 5 is dependent on the depth, size and contour of the crack to be grown. An electromagnetic power source 8 is controlled by a timing and/or control switch 9 and feeds current through Wire 7 and electromagnetic coil 11 when switch 9 is closed. One or more air vents 10 prevents air from being trapped in indentation 5.

Although magnetic material backup strip 3 is normally attached by a suitable flexible adhesive, it may be freely located by placing it on top of a spring or by other conventional methods of placement.

Although structural material 1 may either be magnetic or nonmagnetic, it rnust be flexible. This invention requires access to the inside of the nonmagnetic material vessel but not a magnetic material vessel.

In operation, A.C. current is fed through coil 11 (alternatively, DC. current may be fed intermittently to coil 11 such as by opening and closing switch 9 in order to produce an effect similar to AC. current), to exert a force on nonmagnetic structural material 1 by magnetic material backup strip 3 being intermittently attracted toward core 4, thereby inducing a vibration in structural material 1. This causes the desired fatigue crack.

If structural material 1 is magnetic, when coil 11 is energized magnetic material 1 will be intermittently attracted into indentation 5, inducing a vibration therein to cause the desired fatigue crack.

Some of the advantages of this invention are:

(1) The mechanism of this invention may be placed entirely outside of a closed vessel (the structural material) if it is magnetic. However, a magnetic material back-up sheet or wires of minimum size must be employed inside a nonmagnetic structural material;

(2) A magnetic material back-up sheet(s) or wire(s) is used if necessary (with a nonmagnetic structural material) or if desirable with any type of structural material;

(3) This invention exclusively permits the study of a structural material under unique and significant combined stresses. These unique and significant combined stresses result from the simultaneous growth of a designed fatigue crack and the application of one or more other loads. The one or more other loads may be uniaxial loads, biaxial loads, internal pressures or external pressures;

(4) The structural material rnay be either cooled or heated during fatigue crack growth;

(5) This invention is capable of wide variations in duration, cyclic rate, force and/ or amplitude; and

(6) The electromagnetic core of this invention does not require rigid attachment to the material test specimen but only necessary location.

Various other modifications and variations of this invention will become readily apparent to those skilled in the art in the light of the above teachings, which modifications and variations are within the spirit and scope of this invention.

I claim:

1. An apparatus for inducing a fatigue crack in a discrete area of a structural material, comprising hollowed out contacting means; said hollowed out contacting means being contoured to the contour of said material and being in contact with a first surface around said discrete area of said material, and electromagnetic means for magnetizing said contacting means, thereby permitting deflection of part of said material corresponding to said discrete area relative to said hollowed out contacting means in order to induce a fatigue crack in said material.

2. An apparatus as set forth in claim 1 additionally comprising in air vent in said hollowed out contacting means.

3. An apparatus as set forth in claim 1 additionally comprising magnetic means which contact a second surface of said material opposite said first surface, the area of said second surface being less than said discrete area and said material being made of nonmagnetic material.

4. An apparatus as set forth in claim 2 wherein said material is a hollow vessel.

5. An apparatus as set forth in claim 4 wherein said vessel has a crack starter notch in said discrete area.

6. An apparatus as set forth inclairn 5 additionally comprising an air vent in said hollowed out contacting means.

7. An apparatus for inducing a fatigue crack in a discrete area of a generally cylindrical vessel, comprising an electromagnetic core having a hollowed out tip at one end, said tip having a contoured contacting surface that corresponds to the contour of the generally cylindrical vessel and a vent passage in said tip for venting the hollow of said tip; and electromagnetic means for magnetizing said core and tip so that when said tip is placed in contact with the descrete area, a fatigue crack can be induced in the vessel at discrete area by being forced into said hollowed out tip.

References Cited FOREIGN PATENTS 779,262 10/ 1934 France. 147,834 1960 U.S.S.R. 167,054 1964 U.S.S.R.

JERRY W. MYRACLE, Primary Examiner U.S. Cl. X.R. 

